I have been trying to improve my poor fuel mileage (12-14 mpg) on my stock 95 Country. I've replaced the cracked header with a non cracked one, replaced the original looking fuel filter with a new one, full tune up with plugs, wires, rotor, cap and air filter, new O2 sensor, throttle body and IAC fully cleaned (carb dipped)

Visually inspected all vacuum lines and Checked for vacuum leaks with an unlit propane torch on low and found none. I'm currently building a smoke tester but the vehicle exhibts no other vacuum leak symptoms other than poor fuel mileage.

Today I tested the coolant temp sensor (in the thermostat housing) and the intake air temp sensor and bounced my readings off of the chart below. Temp readings were taken with an IR heat gun on the thermostat housing and the sensor bodies themselves (so may be off a little one way or another but should be pretty close) and OHM readings were pulled with a digital FLUKE multimeter.

Thermostat housing sensor
212 degrees shows 960 ohms. Based on the chart this is feeding the ECU with lower than actual engine coolant temps.

Intake air temp sensor measured 150 degrees and read 2,300 ohms which again going off the chart shows to be feeding incorrect data to the ECU showing lower than actual intake temps.

I'm going to call the parts department tomorrow since I'd prefer to have Mopar sensors (they are pretty good on parts pricing) and see if 1)they can still get the sensors and 2)how much they cost but before I drop the money on these can anyone pick apart my troubleshooting other than not testing them in a pot of hot water vs using an IR gun while mounted to the Jeep?

As far as I can find the above values apply to both the coolant temp sensor and the intake air temp sensor


Cross posted on NAXJA and JeepForum

 

The intake air temp and the intake manifold temp will diverge by a large margin so IR sampling of the manifold to gauge the airflow temp inside it won't give you what you're looking for. You will need a thermocouple probe (K-type) for that. Your Fluke is probably equipped for that already (yellow rectangular receptacle with two slots of different lengths). The unused fitting normally capped off on the manifold can be used to insert the thermocouple probe. Get a rubber cork of suitable size and open up a hole in the center with a drill bit just large enough to route the end of the thermocouple probe through it. Then force the plug down onto the uncapped fitting to get the tip of the probe into the air chamber of the manifold. This arrangement should give you an accurate temp reading without sacrificing vacuum.

IR sampling of the t-stat housing is also problematic. The external surface of the metal versus the actual coolant temp will present a differential possibly unusable to qualify your tests. There's another thermocouple in the form of a flexible wire that can be inserted into the water itself and give a fairer sampling. Remove the radiator cap, insert the wire and replace the cap. The cross section of the thermocouple wire (J-type) is pretty small (1/16th inch) and shouldn't cause the cap to leak.

https://www.fluke.com/en-us/product/.../fluke-80pk-24
https://www.fluke.com/en-us/product/...s/fluke-80pj-1

Onto your mileage, there's another poor mileage story/fix and link below -

" reviving a post from the dead. I do believe i fixed my issue once and for all i noticed i had a lower end on the vacuum pressure on my engine so this took months to find until today lol i poked my head under my jeep on a lift and doing an inspection im looking and noticed a rubber line has rotted away going from the gas tank to the evap charcoal canister so idk how i missed this but i stuck a new line on it and omg my jeep feels like a muscle truck i got alot of power back and i do believe my mpg will also fix itself with this too i need to do a vacuum test to confirm but this seems like a right step in the path lol."

https://www.cherokeeforum.com/f2/gas...ht=bad+mileage

 

So i let everything cool to ambient temps and checked again and at this point the readings are close enough both hot and cold to what the FSM says (different from what I originally posted see pic below) that I'm considering my coolant and temp sensors good to go. Even using the IR heat gun they're within specs or within a few hundred ohms tops.

I have went back to inspecting all the vacuum and evap lines closer. I found a patch I did on the vacuum line going from the fuel regulator to the intake manifold to be very loose and leaking. That has now been fixed. I ran my small propane torch (unlit) over all the junctions and lines I could find and got no response from the engine at any time but I'll still be again visually inspecting all the lines, and try to get my homemade smoke tester done today and run a smoke test tomorrow.

 

Those numbers certainly are not horrible (to be expected actually on a smogged out 24 year old tractor engine) and hence why so many were clunked for cash.

 

This is my 4th XJ and I've never had mileage this bad even on my lifted ones with larger than stock tires. This is an Upcountry height XJ with 235s. People with 35s are getting better mileage than me. The only reason in my experience that an XJ, especially an almost stock one, wouldn't be getting upper teens or even 20MPG on the highway is if something is wrong with it.

Don't even get me started on that disgrace of a program Cash for perfectly running vehicles.

 

My 95 with 235's easily gets high teens average mpg, it'll get an average of 20 with 60/40 highway/city miles driving conservatively and without extra idling.

I completely agree on the insane program to destroy perfectly good engines because Barry and his buddies hate cars.

The worst part is - a brand new 2019 JL 4-door gets 17/23... so for all the wahhh crying about the 4.0 I6, even with all the fancy VVT and ESS features, a brand new Wrangler doesn't get any better gas mileage than my almost 24 year old XJ.